fix field default values

test/cases/module_def/module_ops.tst

@@ -1,6 +1,6 @@
 module Opers
   int foo(int x)
-    int abba = x + self.index
+    int abba = self.index
     return abba + 5
   end
 end
@@ -12,16 +12,12 @@ s(:expressions,
       s(:parameters,
         s(:parameter, :int, :x)),
       s(:expressions,
-        s(:name, :int),
-        s(:assign,
-          s(:name, :abba),
-          s(:operator, "+",
-            s(:name, :x),
-            s(:field_access,
-              s(:receiver,
-                s(:name, :self)),
-              s(:field,
-                s(:name, :index))))),
+        s(:field_def, :int, :abba,
+          s(:field_access,
+            s(:receiver,
+              s(:name, :self)),
+            s(:field,
+              s(:name, :index)))),
         s(:return,
           s(:operator, "+",
             s(:name, :abba),